Tip 0387 has a solution for when the PDC comes online.

On the BDC that you want to promote, use Regedt32 to navigate to:

HKEY_LOCAL_MACHINE\Security

Choose Security / Permissions and insure that Administrators have Full Control on this key and all sub-keys. Navigate to:

HKEY_LOCAL_MACHINE\Security\Policy\PolSrvRo

Change the <no name> value entry from 02000000 to 03000000
where the 3 indicates Primary and the 2 indicates Backup.

Shutdown and restart Windows NT.